This invention relates to enediyne compounds and conjugates thereof, methods for making and using such compounds and conjugates, and compositions comprising such compounds and conjugates.
The enediynes are a family of antibiotics that possess a distinctive strained nine- or ten-member ring system comprising a Z-carbon-carbon double bond and two carbon-carbon triple bonds, usually arranged with the latter two flanking the former. The enediynes are potent damagers of DNA, causing single and double strand cuts. Their potency is attributed to their ability to bind to DNA and undergo a Bergmann rearrangement in which the strained ring system is converted into a highly reactive 1,4-benzenoid diradical, which damages the DNA by abstracting hydrogens from it.

Uncialamycin is an enediyne isolated from a Streptomyces strain found on the lichen Cladonia uncialis (Davies et al. 2005; 2007). (Full citations for references cited in this specification by first named author or inventor and year are provided in the section entitled “REFERENCES” later herein.)

The structure of uncialamycin has been confirmed by total synthesis (Nicolaou et al. 2007a; 2007b). In the course of the synthesis, it was noted that the unnatural 26(S) epimer was almost as active as the natural 26(R) epimer—that is, the stereochemistry of the C27 methyl had a minor effect on biological activity. Both epimers were active against several ovarian tumor cell lines. The IC50 values ranged from 9×10−12 to 1×10−10, depending on the epimer and cell line or sub-line (Nicolaou et al., 2008).
Conjugates are an important method for the delivery of anti-cancer drugs, which are often highly cytotoxic and might otherwise be problematic to administer due to the risk of systemic toxicity. In a conjugate, the drug is conjugated (covalently linked) to a targeting moiety that specifically or preferentially binds to a chemical entity characteristic of the cancer cell, thus delivering the drug there with high specificity. Further, the drug is held in an inactive form until released from the conjugate, usually by cleavage of the covalent linker.
Typically, the targeting moiety is an antibody or an antigen-binding portion thereof, whose antigen is overexpressed or uniquely expressed by a cancer cell (“tumor associated antigen”). In such instances, the resulting conjugate is sometimes referred to as an “immunoconjugate” or an “antibody-drug conjugate” (ADC). Preferably the tumor associated antigen is located on the surface of the cancer cell, but also can be one that is secreted into the vicinal extracellular space. Upon binding, the antigen-conjugate complex is internalized and eventually finds its way inside a vesicular body such as a lysosome, where the covalent linker is cleaved, liberating active drug to exert its chemotherapeutic effect.
Advantageously, the covalent linker is designed such that cleavage is caused by a factor prevalent inside a cancer cell but not in plasma. One such factor is the low lysosomal pH, so that the covalent linker can be an acid-sensitive group such as a hydrazone. Another such factor is the generally higher intracellular concentration of glutathione, allowing for the cleavage of a disulfide covalent linker by a disulfide exchange mechanism. Yet another such factor is the presence of lysosomal enzymes such as cathepsin B, which can cleave peptide linkers designed to be preferred substrates (Dubowchik et al. 2002).
Conjugates have been used to deliver enediyne drugs in oncology. Gemtuzumab ozogamicin (Mylotarg®) is a conjugate of an anti-CD33 monoclonal antibody and a derivative of the enediyne calicheamicin. It was approved for treatment of acute myelogenous leukemia but was later withdrawn from the market. Several other enediyne drugs, especially in the conjugated form, have been the subject of development efforts. For a review, see Shao 2008.